The parameters argc and argv are the argument count and argument array as passed to main() (see exec() ). The argument optstring is a string of recognized
option characters; if a character is followed by a colon, the option takes an argument. All option characters allowed by Utility
Syntax Guideline 3 are allowed in optstring. The implementation may accept other characters as an extension.

The variable optind is the index of the next element of the argv[] vector to be processed. It shall be initialized
to 1 by the system, and getopt() shall update it when it finishes with each element of argv[]. When an element of
argv[] contains multiple option characters, it is unspecified how getopt() determines which options have already been
processed.

The getopt() function shall return the next option character (if one is found) from argv that matches a character
in optstring, if there is one that matches. If the option takes an argument, getopt() shall set the variable
optarg to point to the option-argument as follows:

If the option was the last character in the string pointed to by an element of argv, then optarg shall contain the
next element of argv, and optind shall be incremented by 2. If the resulting value of optind is greater than
argc, this indicates a missing option-argument, and getopt() shall return an error indication.

Otherwise, optarg shall point to the string following the option character in that element of argv, and
optind shall be incremented by 1.

If, when getopt() is called:

argv[optind] is a null pointer
*argv[optind] is not the character - argv[optind] points to the string "-"

getopt() shall return -1 without changing optind. If:

argv[optind] points to the string "--"

getopt() shall return -1 after incrementing optind.

If getopt() encounters an option character that is not contained in optstring, it shall return the question-mark (
'?' ) character. If it detects a missing option-argument, it shall return the colon character ( ':' ) if the
first character of optstring was a colon, or a question-mark character ( '?' ) otherwise. In either case,
getopt() shall set the variable optopt to the option character that caused the error. If the application has not set
the variable opterr to 0 and the first character of optstring is not a colon, getopt() shall also print a
diagnostic message to stderr in the format specified for the getopts
utility.

The getopt() function need not be reentrant. A function that is not required to be reentrant is not required to be
thread-safe.

RETURN VALUE

The getopt() function shall return the next option character specified on the command line.

A colon ( ':' ) shall be returned if getopt() detects a missing argument and the first character of
optstring was a colon ( ':' ).

A question mark ( '?' ) shall be returned if getopt() encounters an option character not in optstring or
detects a missing argument and the first character of optstring was not a colon ( ':' ).

Otherwise, getopt() shall return -1 when all command line options are parsed.

ERRORS

No errors are defined.

The following sections are informative.

EXAMPLES

Parsing Command Line Options

The following code fragment shows how you might process the arguments for a utility that can take the mutually-exclusive options
a and b and the options f and o, both of which require arguments:

APPLICATION USAGE

The getopt() function is only required to support option characters included in Utility Syntax Guideline 3. Many
historical implementations of getopt() support other characters as options. This is an allowed extension, but applications
that use extensions are not maximally portable. Note that support for multi-byte option characters is only possible when such
characters can be represented as type int.

RATIONALE

The optopt variable represents historical practice and allows the application to obtain the identity of the invalid
option.

The description has been written to make it clear that getopt(), like the getopts utility, deals with option-arguments whether separated from the option by
<blank>s or not. Note that the requirements on getopt() and getopts are
more stringent than the Utility Syntax Guidelines.

The getopt() function shall return -1, rather than EOF, so that <stdio.h> is not required.

The special significance of a colon as the first character of optstring makes getopt() consistent with the getopts utility. It allows an application to make a distinction between a missing
argument and an incorrect option letter without having to examine the option letter. It is true that a missing argument can only be
detected in one case, but that is a case that has to be considered.